1. Field of the Invention
The present invention relates to a zoom lens bearing camera, and particularly relates to a camera provided with a zoom lens having a full-aperture F number which changes as the focal length is changed, the camera being capable of using an electronic flash as an auxiliary light source, and having an automatic magnification changing function so as to automatically adjust the focal length of the zoom lens correspondingly to the subject distance.
2. Description of the Related Art
As disclosed in Japanese Unexamined Patent Publication No. Hei-1-232315 discloses a Zoom lens bearing camera having an automatic magnification changing function, in which subject distances are measured with respect to at least three points in a photographic angle of view, and the size of the subject occupying the photographic angle of view is judged on the basis of the result of the subject distance measurement, so that the magnification of the zoom lens is changed so as to obtain an optimum zoom focal length in accordance with a preset program.
That is, in such a camera, the subject distances are measured with respect to at least three points, for example, center, right and left points, in a photographic angle of view, and judgment is made as to which one of those three points gives the shortest distance. By use of the photographic focal length of the lens corresponding to this shortest distance and the depth of field in this photographic focal length, the size of the subject is judged on the basis of whether the measured distances with respect to the other points are within this depth of field, and the focal length of the zoom lens is determined in accordance with a predetermined magnification-changing program on the basis of the result of judgment of the size.
In a camera in which the subject distance measurement is performed not with respect to a plurality of points as in the above-mentioned camera, but with respect to only one point, the focal length of a zoom lens is determined simply on the basis of the distance to the subject and an assumed size of the subject in accordance with a variable-magnification program for an upper half-length portrait mode, a full-length portrait mode, etc.
In those methods, there is no special problem when the brightness of a scene including the subject is satisfactory, but if it is dark enough to need to use an electronic flash, underexposure may result, since the focal length of the zoom lens is determined independent of the range of the electronic flash.
Additionally, Japanese Unexamined Patent Publication No. Hei-2-62510 discloses another camera in which, with respect to a zoom focal length set by a photographer in advance, judgment is made as to whether or not underexposure will occur on the basis of the full-aperture F number of the lens in the preset zoom position, the subject distance, and the radiation quantity (guide number: GNo) of an electronic flash, and if there is a possibility of occurrence of underexposure, the lens is automatically zoomed to the wide angle side having a small full-aperture F number.
That is, at the focal length of the zoom lens set by the photographer, it is judged whether or not underexposure will occur on the basis of the effective F number of the lens in the zooming position, the brightness in the photographic angle of view, the subject distance, and the GNo of the electronic flash, and the lens is automatically zoomed to the wide angle side only when it has been proved that underexposure will occur. The above-mentioned judgment is performed every time when zooming drive to the wide angle side is performed in one step, and the zooming of the zoom lens is stopped at the position of the focal length in which the possibility of occurrence of underexposure is removed. If the possibility of occurrence of underexposure is not removed by the zooming to the end of the wide angle side, an alarm signal is produced.
In the above-mentioned camera, it is possible to prevent underexposure from occurring, however, the angle of view may be changed automatically and the intention of the photographer set aside. This is because of a preference, built into the camera, for correct exposure over angle of view.
As has been described above, in a camera equipped with such an automatic magnification changing device, having a zoom lens in which a maximum-aperture F number of a diaphragm (full-aperture F number) changes between its wide angle side and its telephoto side, there is no special problem when the scene is bright, but various problems occur as mentioned above if the scene is so dark as to necessitate use of an electronic flash. This is because the focal length is set by optimizing the angle of view while disregarding the range of the electronic flash.
The guide number of an electronic flash is given by the equation GNo=(range).times.(F number). Accordingly, the diaphragm is used in its fully opened state when a subject having a large range (long distance) is to be taken. In a zoom lens having full-aperture F numbers which differ between its wide angle side and its telephoto side, the longest range of electronic flash on the wide angle side is different from that on the telephoto side. In other words, in the case of electronic flash photography at a certain photographic distance, it is possible for underexposure to occur on the telephoto side while proper exposure is obtained on the wide angle side.